When a liquid crystal compound having a polymerizable functional group (hereinafter referred to as a polymerizable liquid crystal compound) or a liquid crystal composition containing at least one kind of such a polymerizable liquid crystal compound (hereinafter referred to as a polymerizable liquid crystal composition) is irradiated with active energy rays such as ultraviolet rays while being in an oriented liquid crystalline phase, a polymer in which the liquid crystal molecules are fixed in their oriented state can be obtained. The polymer thus obtained exhibits anisotropy in physical properties such as refractive index, dielectric constant, magnetic susceptibility, elastic modulus, and thermal expansion coefficient and is therefore applicable as an optically anisotropic material, such as a retardation film, a polarizer, a polarizing prism, a luminance improving film, a low pass filter, various optical filters, and a covering of optical fibers. It is important for the optically anisotropic material (polymer) obtained by the polymerization to have not only the anisotropy but other characteristics such as polymerization rate, transparency after polymerization, mechanical strength, coating properties, solubility, crystallinity, shrinking properties, water permeability, water absorption, melting point, glass transition point, clear point, chemical resistance, and heat resistance.
Liquid crystals useful as the optically anisotropic material described include cholesteric liquid crystals showing specific liquid crystalline properties attributed to the helical structure of molecular alignment. Cholesteric liquid crystals exhibit selective reflection of light such that, when natural light enters in a direction parallel with the helical axis, approximately one half of light of a certain wavelength band is reflected in the form of right-handed (or left-handed) circularly polarized light and approximately another half of the light is transmitted in the form of left-handed (or right-handed) circularly polarized light. The bandwidth Δλ of the light selectively reflected by the cholesteric liquid crystals is represented by Δn·P where Δn is an optical (refractive index) anisotropy and P is the pitch of the helical structure. The pitch P is a specific value decided by the liquid crystal molecular structure. The larger the Δn, the broader the Δλ, namely, the broader the wavelength range in which the selective reflection occurs.
Retardation R that affects contrast of optical anisotropy is represented by Δn·d where Δn is an optical (refractive index) anisotropy and d is a film thickness. Because R must be set at a specific value, making Δn larger results in reduction of d. A smaller thickness of an optically anisotropic film makes it easier to control the liquid crystal orientation in polymerization. This brings about improvement of production yield, leading to improved production efficiency.
Polymerizable liquid crystal compounds having a (meth)acryl group as a polymerizable functional group have high polymerizability and provide polymers with high transparency, which have been studied extensively for use as an optically anisotropic material, as disclosed, e.g., in the patent documents (1) to (9).
Patent Document (1) JP 11-116534A
Patent Document (2) JP 11-1130729A
Patent Document (3) JP 11-513360 A
Patent Document (4) Japanese Patent 3228348
Patent Document (5) JP 2005-015473A
Patent Document (6) JP 2005-206579A
Patent Document (7) JP 2002-265421A
Patent Document (8) JP 2002-308831A
Patent Document (9) JP 2002-308832A
Of the polymerizable liquid crystal compounds with a (meth)acryl group disclosed in the patent documents (1) to (9) cited above, however, those of the documents (1) to (6) cannot be said to be sufficient in characteristics such as liquid crystalline properties, solvent solubility, coating properties, and optical anisotropy. In particular, those showing large optical (refractive index) anisotropy (Δn) have difficulty in providing practical optically anisotropic materials because some of them lack in solubility, coating properties or orientation properties and some others cannot be converted into film. Although the polymerizable liquid crystal compounds of the patent documents (7) to (9) have large Δn values, are easy to apply to an alignment layer, and are easily oriented, they have disadvantages such as incapability of exhibiting a liquid crystal phase at room temperature, instability in film thickness after processed into film, and difficulty in controlling the orientation in film.